1. Field of the Invention
Present invention relates to opto-acoustoelectric devices and method for analyzing mechanical vibration and sound using the opto-acoustoelectric devices.
2. Description of the Related Art
In optical microphones of earlier technology, a diaphragm is stacked on a supporting substrate by some type of tensile force. Because of the mechanical configuration, the resonance frequency of the diaphragm was uniquely determined according to physical parameters such as mass and spring constant. Even though the frequency characteristic of the optical microphone is nominally specified as being flat, the diaphragm's sensitivity drops actually as frequency deviates from the inherent resonant frequency of the diaphragm. A technique of arranging a plurality of diaphragms, each of the diaphragms having different sizes and spring constants, is reported as an approach to realizing a microphone having a broad dynamic range.
On the other hand, the condenser microphone detects the diaphragm's vibrational displacement caused by acoustic pressure (sound pressure) by the variation of electrostatic capacitance. Compared to the condenser microphone, the optical microphone beams light onto a diaphragm. As the diaphragm vibrates in response to sound pressure, changes in the intensity of the reflected light are detected as the oscillating displacement of the diaphragm. Compared to the condenser microphone, the optical microphone is expected to have excellent performance in directivity and noise tolerance attributes.
In the optical microphones of earlier technology however, the diaphragm is exposed to gas laser, solid-state laser, or semiconductor laser diode light output, and changes in the intensity of the reflected light are detected as the oscillating displacement of the diaphragm. However, there are drawbacks inherent in the mechanical structure and the dimensional constraints of the methods proposed in the above-mentioned earlier technology. Also concerning mounting methodology, there are concerns that manufacturing yield would drop due to an increase in constituent components
Also, optical microphones of earlier technology require an incidental-light optics system for guiding incident light onto the diaphragm, and catoptrical system for guiding reflected light to a given photo detector. Because high optical precision is required of these optics systems, optical fiber or optical waveguides are used to make it possible to establish an arbitrary optical path Also, the light beam shifts atop the photodetector's surface in correspondence with the physical displacement of the diaphragm caused by vibration, thus in order to greatly widen the shift width of the reflected light beam, measures such as establishing a lens between the diaphragm and the circuit board, on which the light emitting and detection elements are mounted, are essential. In other words, because the diameter of the light waves reflected from the diaphragm is widened using light focusing methods of optical microphones of earlier technology, great light intensity is essential in order for the photo detector to detect differences in light intensity and sense minute vibrational displacement of the diaphragm.
As a result, optics such as optical path supplementation elements and compensation elements became essential. Because of the optics, aside from the light-emitting elements, the light emitting detection elements and the diaphragm, other auxiliary optical elements and a new supplemental system are required to satisfy the demand of high-precision optical alignment of optics. Therefore, complicated optics of an optical microphone leads to reduced manufactured yield, and the entire cost as a system increases. Also, when optical fiber or optical waveguide is employed, the application range of optical microphones of earlier technology becomes narrow.